In the following description and claims the term "plate" will be used generically to describe objects which qualify under any of the terms plate, foil and sheet. Moreover, where in the following description and claims there is reference to an incidence angle of a focused laser light beam, this is to be understood as meaning the angle between an imaginary axis of the converging portion of the incident beam and the irradiated surface.
Radon-222 is a radioactive gas, which is produced as the decay product of the teressial Radium-226. This radioactive gas is present in the atmosphere, and forms radioactive decay series, including two radio-isotopes (Po-218 and Po-214) which are .alpha.-emitters. The .alpha.-emitters enter the respiratory system in the normal breathing process, thus exposing the respiratory tract to energetic .alpha.-particles.
Radon is widely recognized as the major (&gt;50%) source of ionizing radiation exposure to the general public, and it is estimated that some 10,000 cases of fatal lung cancer are caused annually in the U.S. alone by this carcinogenic agent.
The first step in reducing the exposure to radon is the identification of high-radon locations based on measurements of the radon level. A variety of methods exist for the measurement of radon, which can be classified by the air sampling period, ranging from a few minutes for grab-sampling, via a few days for semi-integrative measurements, and up to a full year for integrative measuring methods.
As the radon level in a given location is known to fluctuate, short sampling periods may yield misleading results, and thus it is widely accepted that the epidemiologically most significant results are obtained by fully-integrative radon-level measurements.
Of the various integrative radon measuring methods, SSNTD is the most prominent one. This method is based on the phenomenon that the passage in matter of an energetic, heavy ionizing particle such as an .alpha.-particle or a proton, results in a high linear energy transfer (LET) to the matter, of the order of 100 MeV/(g/cm.sup.2). This high amount of energy is capable of damaging the chemical bonds in the track of the particle in the exposed material and where such material is a crystalline or polymeric solid body, the damage to the chemical bonds in the track of the particle may result in an observable change in the local properties of the material. One such change is a local reduction in the material's resistance to chemical etching. In consequence the etching process at the tracks of the particles will proceed at a higher rate than at the undamaged sites of the body, which results in the formation of small etching pits at the sites where the ionizing particles entered the body.
Thus SSNTD comprises exposure of a suitable transparent plate to ionizing radiation for the purpose of inducing localized change of the plate followed by chemical etching, and counting the resulting pits.
Conventionally, in the performance of SSNTD the radiation exposure is estimated by observing the density of the radiation-induced etching pits in it. Many methods have been proposed for the counting of these pits, the most advanced of which is the application of a computerized image analyzer. As the application of this method is rather expensive, most current SSNTD techniques are based on human observation and counting of these pits under a microscope.
SSNTD may also be used for the measurement of neutron radiation which may be present in areas that had been afflicted by an atomic explosion or spillage, or in active nuclear reactors.
It is the object of the present invention to provide method and means for a reliable etching pit count in the performance of radiation measurements in gaseous surroundings by SSNTD.